rolling over from back to tummy

This is funny because Isaac rolled over once about a month ago and hasn't really tried since. Two days ago he rolled over from his back to his tummy and then back to his back again. Now every time he is on his back he is trying to roll over. In his crib, on the changing table, on the floor he can't not roll over when given the opportunity. Yesterday I was in a Parent/Baby class and the instructor was doing baby yoga. During every activity Isaac just kept rolling over and I had to keep flipping him back over to do the poses. He is so proud of himself!